1. Field of Invention
The present invention relates generally to magnetic memory and more specifically to a memory device that utilizes the magnetoresistive effect to store binary data.
2. Description of the Related Art
Many advances in memory technology have been made in recent years. One such advance is the magnetic tunnel junction (MTJ) that makes use of the tunneling magnetoresistive effect to store binary information. MTJs are favored because they provide high data read speeds, are nonvolatile, and have a high magnetoresistive ratio. The basic structure of the MTJ is shown in FIG. 1 as tri-layer device having a barrier layer 16 sandwiched between magnetic (ferromagnetic) layers 12 and 14. Each magnetic layer has an associated magnetization direction.
In a typical arrangement, one of the magnetic layers is configured to be a fixed layer 14. An anti-ferromagnetic layer (not shown) may be useful in fixing the magnetization direction of the fixed layer. Thus, the fixed layer is treated substantially like a permanent magnet with a permanent direction. The single headed arrow in layer 14 indicates that layer 14 is a fixed layer with a fixed magnetic direction. A second magnetic layer is termed a free layer 12. The free layer 12 is configured to switch the direction of its magnetization in response to an applied magnetic field of sufficient magnitude. The double headed arrow at free layer 12 indicates that the magnetization direction of the free layer 12 may be inverted by an applied magnetic field.
In order to store binary data, the MTJ must have two possible logical states (i.e. binary states). These states are often referred to as “1” and “0.” The state of the MTJ is defined by whether the magnetization directions of the two magnetic layers 12 and 14 are parallel or anti-parallel. If the magnetization directions the two magnetic layers 12 and 14 are the same then they are said to be parallel. Alternatively, if the magnetization directions of the two magnetic layers are opposite, they are said to be anti-parallel.
The tunneling magnetoresistive effect is based on the phenomena that an applied magnetic field can influence the resistivity of a material. In simple terms, the resistance to a current passing through the MTJ is “high” when the magnetization directions of the magnetic layers are anti-parallel and “low” when the magnetization directions are parallel. Usually, the resistivity of an MTJ is determined by measuring a read current passed perpendicularly through each layer of the MTJ. A read current (i) is shown passing perpendicularly through the layers of FIG. 1. Because of the direction of read current flow, an MTJ is termed a current perpendicular to plane (CPP) device.
One measure for the quality of an MTJ is its magnetoresistive ratio defined as (ΔR/Rmax). ΔR is defined as the difference between the resistivity of the MTJ when the magnetization directions are anti-parallel and the resistivity of the MTJ when the magnetization directions are parallel, while Rmax is the resistivity of the MTJ when the magnetization directions are anti-parallel (maximum resistivity).
When writing to the MTJ, the magnetization direction of the free layer 12 is switched by applying a magnetic field to the MTJ. Usually, a pair of conducting lines running perpendicularly to one another are used to apply the external magnetic field to the MTJ for writing. These lines may be termed a bit line and a digit line. The bit line may also be used to apply the read current.